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  • Title: Substrate specificity of the rat liver Na(+)-bile salt cotransporter in Xenopus laevis oocytes and in CHO cells.
    Author: Schroeder A, Eckhardt U, Stieger B, Tynes R, Schteingart CD, Hofmann AF, Meier PJ, Hagenbuch B.
    Journal: Am J Physiol; 1998 Feb; 274(2):G370-5. PubMed ID: 9486191.
    Abstract:
    It has been proposed that the hepatocellular Na(+)-dependent bile salt uptake system exhibits a broad substrate specificity in intact hepatocytes. In contrast, recent expression studies in mammalian cell lines have suggested that the cloned rat liver Na(+)-taurocholate cotransporting polypeptide (Ntcp) may transport only taurocholate. To characterize its substrate specificity Ntcp was stably transfected into Chinese hamster ovary (CHO) cells. These cells exhibited saturable Na(+)-dependent uptake of [3H]taurocholate [Michaelis constant (K(m)) of approximately 34 microM] that was strongly inhibited by all major bile salts, estrone 3-sulfate, bumetanide, and cyclosporin A. Ntcp cRNA-injected Xenopus laevis oocytes and the transfected CHO cells exhibited saturable Na(+)-dependent uptake of [3H]taurochenodeoxycholate (Km of approximately 5 microM), [3H]tauroursodeoxycholate (Km of approximately 14 microM), and [14C]glycocholate (Km of approximately 27 microM). After induction of gene expression by sodium butyrate, Na(+)-dependent transport of [3H]estrone 3-sulfate (Km of approximately 27 microM) could also be detected in the transfected CHO cells. However, there was no detectable Na(+)-dependent uptake of [3H]bumetanide or [3H]cyclosporin A. These results show that the cloned Ntcp can mediate Na(+)-dependent uptake of all physiological bile salts as well as of the steroid conjugate estrone 3-sulfate. Hence, Ntcp is a multispecific transporter with preference for bile salts and other anionic steroidal compounds.
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